Elimination of trans-unsaturated fatty acid compounds by selective adsorption with zeolites

ABSTRACT

A novel process for the selective elimination of fatty acid compounds containing carbon-carbon double bonds in trans configuration from a substrate containing cis- and trans-isomers of said fatty acid compounds, by selective adsorption by a microporous zeolite material is disclosed. The pore size and shape of usable zeolite materials enable differentiation between cis- and trans-isomers of unsaturated fatty acid chains. The zeolite materials used have a selectivity ratio α trans/cis  higher than 1.00; this ratio is defined based on the elution properties of cis and trans double bond containing fatty acid methylesters dissolved in n-hexane during a column chromatography experiment with the zeolite material as the stationary phase and n-hexane as the mobile phase. Besides selective adsorption of trans-unsaturated fatty acid compounds, simultaneous or subsequent total or partial hydrogenation of the double bonds in said compounds can be carried out while using the same or similar zeolite material, containing finely dispersed catalytic active metals. The majority of these catalytic active sites must be inside the pores.

This application is a 371 of PCT/EP 98/03098 filed May 26, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to the selective elimination oftrans-unsaturated double bonds in fatty acid compounds from a substratecontaining cis- and trans-isomers of said fatty acid compounds. Theelimination takes place by adsorption of said fatty acid compounds byzeolite materials having a selectivity towards trans-isomers. Theinvention also relates to the elimination of trans-isomers of saidcompounds by their selective adsorption by said zeolite materialsfurther containing a metal catalyst, and saturation of the double bondof the fatty acid compounds adsorbed in the pores or cavities of saidzeolite.

The aforementioned trans-unsaturated fatty acid compounds may besaponifiable or non saponifiable molecules. The saponifiable fatty acidcompounds comprise esters, mono-, di- and triglycerides, phospholipids,glycolipids, diol esters of fatty acids, waxes and sterol esters. Thenon saponifiable compounds comprise free fatty acids, sterols,carotenoids, monoterpenes and tocopherols. Other fatty acid compoundswell known for their amphiphilic properties are fatty acid derivativeslike fatty alcohols, fatty amines or fatty acid dimers. The presentinvention relates, in particular, to the elimination oftrans-unsaturated fatty acid residues in triglycerides from edible oilsand fats and is therefore mostly related to food technology.

Oils and fats for food applications are mainly triglycerides: moleculeshaving three fatty acids esterified with glycerol. In most cases, thefatty acid chains are not branched, have a chain length of 4 to 24carbon atoms, and may contain up to three double bonds. Thephysicochemical properties of triglycerides strongly depend on thechemical structure of the fatty acid residues and more particularly ontheir chain length and the amount of double bonds present. In fact, themelting point of triglycerides increases with increasing chain lengthand decreasing unsaturation of the fatty acid residues present.

Hardening by hydrogenation is a common process to increase the meltingprofile of edible oils and fats. In most cases, this process is carriedout as a heterogeneous reaction with hydrogen gas and a heterogeneouscatalyst. Often used catalyst materials are metals like nickel orpalladium finely deposited on carriers like kieselguhr or silica. Thehydrogenation process is typically carried out in agitated batchautoclaves at temperatures above 400 K and hydrogen gas pressures above0.1 MPa. Agitation can be realised by stirring or by more complexsystems like circulating the reactors content through a venturi systemwhere hydrogen is mixed intensively with the oil being hydrogenated.Continuous processes are used as well.

Hydrogenation can be carried out to accomplish complete saturation ofall double bonds present. In many cases, however, partial hydrogenationis aimed for. In the latter case and with the processes actually usedindustrially, isomerisation of carbon-carbon double bonds in the fattyacid residues occurs besides the saturation of double bonds by theaddition of hydrogen. For food applications, starting materials forhydrogenation are of biological origin. In such materials, like palmoil, soybean oil, softseed oils and the like, almost no trans-isomersare present; the position of the double bonds in the fatty acid chainsis well defined too. Hydrogenation by means of metal catalysts likenickel, palladium, platinum, ruthenium, rhodium and others, inevitablyleads to cis/trans-isomerisation, since the reaction mechanism usingsuch catalysts implies a transition state with a freely rotatingsemi-hydrogenated configuration (L. F. Albright, J. Am. Oil Chem. Soc.,40/5, 16 (1963) and G. Cecchi, G. Mallet, E. Ucciani, Riv. Ital. Sost.Grasse, 58/5, 228 (1981) and R. R. Allen, J. Am. Oil Chem. Soc., 63/10,1328 (1986)). The existence of this transition state also leads topositional isomerisation of double bonds when the total addition ofhydrogen is sufficiently slow. Just like saturated fatty acids, suchisomeric fatty acid residues increase also to some extent the meltingprofile of triglycerides.

Although hydrogenation is the main cause for the presence of fatty acidisomers in food oils and fats, similar isomers can be found in otherlipids too. Animal fats like butter fat or tallow have sometrans-isomers, and fully refined, non-hydrogenated fats also may containa very low content of said isomers due to the high temperatureprocessing on refining and deodorisation (L. H. Wesdorp, Lipid Techn.,8/6, 129 (1996)). However, the amount of isomers present in all these issignificantly lower than one can expect in general in hardened products.

According to recent studies, a lot of controversy has been risen aboutpossible health hazards of these trans-unsaturated fatty acids (M. B.Katan, P. L. Zock, R. P. Mensink, Annu. Rev. Nutr., 15, 473, (1995) andBritish Nutrition Foundation, “Trans Fatty Acids”, (1995)).

For that reason, attempts have been made to reduce the trans fatty acidcontent in food products. An important contribution is the reduction ofthese trans fatty acids in hydrogenated edible fats (J. M. Hasman,Inform, 6/11, 1206 (1995)). A lot of hydrogenation process modificationshave been proposed to achieve this objective.

The hydrogenation process of vegetable oils is generally carried out inan agitated batch autoclave. Typical process parameters are a hydrogenpressure ranging from 0.1 to 0.5 MPa and a hydrogenation temperatureranging from 400 to 475 K. Since isomerisation depends on theconcentration and lifetime of the so-called semi-hydrogenated transitionstate, a first approach to reduce isomerisation relies on increasing thehydrogen concentration on the catalyst active sites. This can typicallybe realised by a higher pressure of hydrogen gas supplied, to increaseits solubility in the oil, and by increasing the hydrogen mass transfercoefficient, by more efficient agitation (P. R. Puri, J. Am. Oil Chem.Soc., 55/12, 865 (1978) and J. W. E. Coenen, Riv. Ital Sost. Grasse,58/9, 445 (1981)). Similarly, a reduction of the reaction temperaturehas been proven to have some effect on the isomerisation of doublebonds, more specifically a suppression of trans double bond formation,but also brings along a reduced reaction velocity. Both means, increaseof hydrogen concentration and temperature lowering, although effectiveto some extent in lowering the concentration of isomerised products inhardened oils and fats, can not eliminate isomers, mainly transisomers,in said oils and fats.

A second approach one has followed to reduce trans-unsaturated fattyacid compounds is to influence the catalytic system itself The metalparticles commonly used are as small as 20-50 Å in order to provide ahigh reaction surface area. To modify the catalytic properties of themetallic catalyst, alloying (P. N. Rylander, J. Am. Oil Chem. Soc., 47,482 (1970) and A. I. Thomson, J. Chem. Tech. Biotech., 37, 257 (1987)and J. D. Parry, J. Chem. Tech. Biotech, 50, 81 (1991)) or addition ofmodifiers like amine or ammonium compounds (U.S. Pat. No. 4,307,026 andU.S. Pat. No. 4,228,088 and EP-A-0,576,477 and E. Draguez de Hault, J.Am. Oil Chem. Soc., 65, 195, (1984)) have been used to decrease theisomerisation effects like trans double bond formation.

Still other means have been used to influence the concentration oftrans-unsaturated fatty acid compounds. Homogeneous catalysis with metalcomplexes like benzoate-Cr(CO)₃ or triphenylphosphine complexes ofruthenium or rhodium has been investigated on small scale (E. N.Frankel, J. Am. Chem. Soc., 90, 2446, (1968) and C. Bello, Ibid., 62,1587, (1985) and E. A. Emken, J. Am. Oil Chem. Soc., 65, 373, (1988)). Areduction of the trans fatty acid formation upon hydrogenation wasrealised. Industrial use, however, can not be expected since thecatalysts used, could be toxic and could not be removed easily andeconomically after hydrogenation. Attempts to heterogenise the catalyticsystems mentioned were not successful

Besides changing of the hydrogenation process parameters, use ofmodifiers, alteration of the catalytic metal function and specificsupporting of the metal to decrease the trans-isomeric fatty acidcontent upon hydrogenation, has been studied. Supporting the metal ondifferent materials like titaniumdioxide and kieselguhr has beenpublished (E. Draguez de Hault, J. Am. Oil Chem. Soc., 65, 195 (1984). Ahigh dispersion of the metal in a porous structure has been accomplishedaccording to EP-A-0,233,642, U.S. Pat. No. 4,584,139 and U.S. Pat. No.5,492,877. However, none of these could eliminate trans-unsaturatedfatty compounds from the substrates studied completely.

Still further attempts have been carried out to influence thecis/trans-isomerisation during hydrogenation. Electrocatalytichydrogenation by the addition of hydrogen donors, although decreasingisomerisation, still produced trans-isomers; in addition, the velocityof the reaction was lower (EP-A-0,429,995 and U.S. Pat. No. 4,399,007and G. J. Yusem, J. Am. Oil Chem. Soc.).

Hydrogenation of fats and oils on zeolites has been reported by Koritala(S. Koritala, J. Am. Oil Chem. Soc., 45, 197, (1968)). Pt/Na—Y zeolitewas found to be able to hydrogenate triglyceride samples;cis/trans-isomerisation, however, could not be eliminated (A. Brehm andH. M. Polka, Chem.-Ing. Tech., 61, 963, (1989)). Pd/CuO/ZnO/ZSM-5 hasbeen used to hydrogenate methyllinoleate, although with low conversion,without any trans-isomer formation; this catalyst wasn't very active forthe hydrogenation of triglycerides (R. Müller, “Selektieve Hydrierungvon Ölen an suspendierten Katalysatoren”, MSc thesis, Oldenburg,(1991)). The authors report on the introduction of a hydrogenationselectivity for a palladium loaded zeolite material by havingcopperoxide and zincoxide inside the pores as well. During thepreparation of said metal loaded zeolite material, after the oxidationstep, a subsequent reduction below 473 K is required to have acatalytically active palladium while the oxides of copper and zinc,which are required to obtain the selectivity, survive this reductiveoperation. The elimination of trans isomers with this metal alloy loadedcatalyst, however, can not be attributed to selective adsorption of saidtrans fatty acid isomers, but by excluding formation oftrans-unsaturated fatty acid compounds. The effect, also according tothe authors, must be the result of the restricted mobility of thesemi-hydrogenated transition state when present in the pores of thezeolite material. To increase activity, they recommend to widen thepores of the zeolites. This, however, will certainly not increase theselectivity for the adsorption of trans fatty acid compounds.

It should be clear that the above mentioned methods to avoidisomerisation, mainly cis/trans-isomerisation, are just partialsolutions for what one should aim for: a substrate without any residualtrans-unsaturated fatty acid compounds left. The present invention willovercome all drawbacks of the prior art in providing a process for theadsorption of transunsaturated fatty acid compounds from substratescontaining cis- and trans-isomers of said compounds by a microporouszeolite material having a selectivity for the adsorption of saidtrans-unsaturated compounds bigger than one.

OBJECT OF THE INVENTION

It is the object of the present invention to provide a process thatallows the complete elimination of trans unsaturated fatty acidcompounds from a substrate containing cis and trans isomers of saidfatty acid compounds by means of a microporous zeolite material having aselectivity α_(trans/cis) higher than one. This and other objects andadvantages of the present invention will become apparent as thedescription of the invention proceeds.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention a process is provided to eliminatetrans-unsaturated fatty acid compounds from a substrate containing cis-and trans-isomers of said fatty acid compounds, in which process thetrans-unsaturated fatty acid compounds are selectively adsorbed by amicroporous zeolite material.

In general zeolites are crystalline aluminosilicates in which the threecomponents aluminium, silicon and oxygen are arranged in a fixed,dimensional framework with cavities and pores of uniform size and shape.The zeolite network is composed of SiO₄ and AlO₄ tetrahedra in which thenegative charge on the latter is neutralised by cations like metal ions,ammonium ions or alkali metal ions.

A general formula for an aluminosilicate based zeolite can be writtenas:

M_(x/n) ^(n+)(AlO₂ ⁻)_(x)(SiO₂)_(y)z.H₂ O

where n is the valence of the charge compensating cation M and the x/yratio is smaller than or equal to one according to the Loewenstein rule(D. W. Breck, “Zeolite Molecular Sieves”, J. Wiley and Sons, 1974).

Zeolites can be synthesised with different topologies giving rise topores with different size, shape and dimensionality. The aperture sizesof these micropores generally ranges between 0.4 and 0.8 nm, dependingon the number of tetrahedra in the ring that member them. The dimensionsof the pore apertures manage the accessibility of the internal volume ofthe zeolite by excluding molecules with dimensions exceeding those ofthe aperture of the zeolite pores. Zeolites with a three-dimensionalnetwork of pores are particularly interesting in this invention forobtaining a large accessibility of the substrate molecules and a highpotential metal dispersion (P. B. Weisz and V. J. Frilette, J. Phys.Chem., 64, 342 (1960)).

Three different molecular shape selective effects are known in zeoliteliterature. The first, reactant shape selectivity, appears in theselectivity of the aperture of the zeolite pore in admittingpreferentially one molecule with a typical size or shape out of amixture of molecules. The selective hydrogenation of unbranchedα-olefins out of a mixture containing branched analogues on Pt/ZSM-5 iswell known in the art (P. B. Weisz, Chemtech, 3, 498, (1973) and R. M.Dessau, J. Catal., 89, 520, (1984)). The selective hydrogenation oftrans-2-butene out of a cis/trans-mixture on Pt/A is known in literature(N. Y. Chen, P. B. Weisz, Chem. Eng. Prog Symp. Ser., 73, 86, (1967)).

A second type of selectivity is defined as product shape selectivity:the product of a reaction catalysed by zeolites must have a specificsize or configuration to have the possibility to migrate out of thepores and cages of the zeolite material used. As an example, productselectivity has been shown by the absence of branched products likeisobutane and isopentane in the cracking of n-alkanes on 8-membered ringzeolites.

The third type of selectivity is the transition state shape selectivity.The zeolite pore size and shape control the permissible size and shapeof the transition state between reactant and product molecules. Anexample is the cracking of n-hexane out of a mixture of n-hexane and3-methyl-pentane on H-ZSM-5 (N. Y. Chen, W. E. Garwood, J. Catal., 52,453, (1978)).

We have extensively studied and selected, out of a large number ofdifferent zeolite materials, those that permit the lineartrans-unsaturated fatty acid chains of fatty acid compounds to enterinside the zeolite pores while simultaneously limiting or excluding theaccess of those pores by the bended cis-unsaturated fatty acid chains ofsaid compounds. Typical zeolite materials having this selectivity aremicroporous and have a 10-membered ring structure. They preferably havea high Si/Me³⁺-ratio of their chemical composition, even more preferablehigher than 39 and most preferable higher than 77.5. A typical exampleis a ZSM-5 zeolite. The Me³⁺in many cases is alumina, but othertrivalent kations like boron or iron are possible candidates as well. Ingeneral Me³⁺represents all trivalent kations be it alumina, others ormixtures thereof. A zeolite material having an extremely highSi/Me³⁺ratio and having the required properties as described in theinvention is silicalite.

The selectivity meant in this description of the invention can bedefined based on a simple but well defined liquid chromatographicmethod. To determine the selectivity of the zeolite structures studied,a model solution of methyloleate and methylelaidate has been introduced.The chromatographic response curves are analysed according to the methodof moments (D. M. Ruthven, “Principles of Adsorption and AdsorptionProcesses”, J. Wiley and sons, (1984)) wherein the first moment isrelated to the adsorption equilibrium constant of the methylester. Thevoidage of the adsorbent column was calculated based on the fractionalmicropore volume of the zeolite crystals (D. W. Breck, “ZeoliteMolecular Sieves”, J. Wiley and sons, (1974)). For a packed column ofzeolite adsorbent the first moment of the response curve is denoted as:${\mu \equiv \quad \frac{\int_{0}^{\infty}{c\quad*t\quad*\quad {t}}}{\int_{0}^{\infty}{{c\quad}^{*}\quad {t}}}} = {\frac{L}{ɛ\quad*v}*\quad \left\lbrack {ɛ + {\left( {1 - ɛ} \right)\quad*K}} \right\rbrack}$

wherein μ is the mean of the response curve (s), c the concentration ofmethyloleate and methylelaidate respectively in the fluid phase(moles/cm³), t the time (s), L the length of the adsorbent column (cm),ε the voidage of the adsorbent bed, v the interstitial liquid velocity(cm/s) and K the adsorption equilibrium constant [(moles/cm³crystal)/(moles/cm³ solution)]. α is the separation factor (D. D. Do, P.L. J. Mayfield, American Institute of Chemical Engineers Journal, 33,1397 (1987)) or the selectivity ratio between methylelaidate andmethyloleate and is hereby denoted as:$\alpha_{{trans}/{cis}} = \frac{K_{trans}}{K_{cis}}$

On studying different zeolites, surprisingly we found those zeoliteshaving a selectivity ratio α_(trans/cis), as defined and determined bythe described method, being higher than one, to adsorb selectivelytrans-unsaturated fatty acid compounds out of a substrate containingcis- and trans- isomers of said compounds. Zeolites having a selectivityratio α_(trans/cis) higher than 1.10 have been found to be even moreperformant to selectively adsorb trans-unsaturated fatty compounds outof a substrate containing cis- and trans- isomers of said compounds.

The concentration of trans-unsaturated fatty acid compounds in thesubstrate and the ratio of cis- and trans-isomers in it can differwidely. For substrates with a low concentration of trans-unsaturatedfatty acid compounds, a specific way to carry out the process describedin the present invention, is by simple adsorption and subsequent removalof the microporous zeolite adsorbent loaded with the trans-unsaturatedfatty acid compounds, leaving a mixture exempt of said compounds.Purification of fully refined edible oils that contains sometriglycerides with trans double bonds due to a high temperaturedesodorisation treatment, can be carried out according to this specificadsorption process. Soybean oil, rapeseed oil, palm oil and others aresuitable substrates, but other oils from vegetable or animal origin canbe treated similarly. The adsorption process can be carried outbatchwise or continuously by using a column reactor filled with theappropriate zeolite adsorbent.

For substrates having high levels of trans-unsaturated fatty acidcompounds, the just explained process variant to eliminate saidcompounds is not attractive because of low yield and high costs.

By depositing inside the pores of such zeolite materials, metals likenickel, platinum, palladium, ruthenium, rhodium, cobalt, copper ormixtures thereof and having catalytic activity for the hydrogenation ofcarbon-carbon double bonds, we further surprisingly found to have meansto produce hydrogenated fatty acid containing compounds withouttransisomers. To achieve the latter a two step hydrogenation process canbe used. In that case, fatty acid containing compounds hydrogenated bymethods described in the state of the art and thus undoubtedlycontaining trans-unsaturated fatty acid compounds, are treatedconsecutively or simultaneously with zeolites according to the presentinvention. Complete elimination of trans-isomers in hardened products isthus obtained.

Substrates to be used for the process described in the present inventioncan be mixtures of cis- and trans-unsaturated fatty acid compounds withor without solvents. When solvents are used, a higher adsorption rate isreached by lowering the viscosity of the substrate comprising the transfatty acid containing compounds. Fatty acid containing compounds can bemethylesters as used in the method to determine the selectivity ratio ofzeolite materials for the adsorption of trans-unsaturated fatty acidcompounds. Other compounds are used for food applications liketriglycerides present in edible oils and fats be it partiallyhydrogenated or not, and di- or monoglycerides used as emulsifiers.Still other substrates containing trans- unsaturated fatty acidcompounds for similar or other applications can also be treatedaccording to the present invention. Said compounds can be sugar estersof fatty acids, polyol fatty acid esters, fatty alcohols, fatty amines,dimeric fatty acids, waxes and others being derivatives of fatty acids.

The microporous zeolite with the required adsorptive properties can beregenerated after use in adsorption by leaching with solvent or othermethods like incineration in specific processes. Said zeolites alsocontaining catalytic metals and therefor used for the production ofhydrogenated products containing no trans-unsaturated fatty acidscompounds, could be reused as such, or if necessary after reactivatingby appropriate methods like decoking and reduction.

The invention will now be described further by the following section onmaterials and methods and illustrative but non-limiting examples.

Materials and Methods

Zeolite Y (PY-43) is obtained from Uetikon (CU Chemie Uetikon AG,CH-8707 Uetikon) and ZSM-5 (CBV-8020 and CBV-1502) is supplied by PQ(1700 Kansas Avenue, Kans. 66105-1198). ZSM-22 and ZK-5 are synthesisedaccording to published procedures (P. A. Jacobs, J. A. Martens, StudSurf. Sci. Catal, 33, Elsevier, (1987) and W. M. Meier, G. T. Kokotailo,Z. Kristallogr., 121, 211, (1965)). Calcined zeolite Y and ZSM-5 aretwice ion exchanged overnight with a 1.0 M aqueous solution of NaCl,typically 300 ml solution for 1 g of catalyst to accomplish a completesodium form of the zeolite. The Na-zeolite thus obtained is centrifuged,washed till Cl-free upon addition of a 0.01 N solution of AgNO₃ to thefiltrate and dried at 393 K during 3 to 4 hours until constant weight.Pt-zeolite is prepared by ion exchange with Pt(NH₃)₄Cl₂. The ionexchange is carried out with 1025.2 ml of a 1.10⁻⁴ M solution ofPt(NH₃)₄Cl₂ for 2 g of Na-zeolite. The Pt-zeolite thus obtained isfiltered, washed till Cl-free and dried at room temperature. Calcinationof the catalyst is performed by heating at about 0.5 K/min to 623 K forat least one hour under a flow of oxygen, hereby decomposing thePt-amine complex. After cooling down the sample, the Pt(II) is reducedto the metal state by heating at 0.5 K/min to 773 K for one hour under aflow of hydrogen. The acid sites inherently formed by reducing thePt(II) to the metal state are neutralised under a flow of ammonia at 473K for at least one hour. The hereby formed ammonium ions are exchangedwith sodium according to the aforementioned method. The obtained zeolitecatalyst contains 1 wt. % of platinum.

A second series of samples is prepared. Calcined zeolite ZK-5 and ZSM-5are ion exchanged with a 0.1 M aqueous solution of NH4Cl, typically 100ml solution for 1 g of catalyst, under reflux for 4 hours. The herebyobtained NH₄-zeolite is filtered, washed till Cl-free and calcined at 1K/min to 673 K for a least one hour to remove the ammonia from thecatalyst medium. The acid form of the zeolite powder is mixed withcalculated amounts of PtCl₂, typically 13.63 mg of PtCl₂ to 1 g ofcatalyst, under inert atmosphere to avoid hydrolysis of the metalchloride and hydration of the zeolite. The solid mixture is heated at 5K/min to 823 K under nitrogen atmosphere to perform solid state ionexchange. Hydrogen chloride is formed in situ and purged out of thereactant medium. After cooling down the sample to room temperature, thePt(II) is reduced to its metal state by heating at 0.5 K/min to 673 Kfor one hour under a flow of hydrogen. The acid sites are neutralised bya flow of ammonia and ammonium ions formed are exchanged with sodiumaccording to the aforementioned method. The obtained zeolite samplecontains 1 wt. % of platinum.

The adsorption experiments of methyloleate and methylelaidate areperformed by liquid phase chromatography (HPLC) on a packed bed ofzeolite crystals. The metal column (4.6 mm internal diameter and 48 mmpacked length) contains the zeolite adsorbent enclosed by two 0.5 μmfilters (Alltech, Deerfield, Ill. USA). A liquid chromatography meteringpump (HP1090, Hewlett Packard, Waldbronn, Del.) provides a steady flowof n-hexane through the adsorbent column of packed zeolite crystals. Theeluent is monitored by a refractive index detector (R. I. HP1047A,Hewlett Packard, Waldbronn, Del.). A small column filled with molecularsieve pellets (5A, E. Merck, Darmstadt, Del.) is placed in line betweenthe outlet of the pump and the chromatographic column to dry the mobilephase (n-hexane) continuously, since water could influence theadsorption equilibrium constants. The mobile phase is continuously driedover a packed column of molecular sieve pellets (5A, E. Merck,Darmstadt, Del.) to avoid interaction of water on the adsorptionequilibrium constant. A small pulse (20 μl) of pure (99 wt. %) andmethylelaidate (99 wt. %) (both: Fluka Chemie AG, Buchs, CH) is injectedseparately at time zero and the concentration response at the outlet ofthe column is monitored on the R. I. detector. The adsorptionmeasurements are performed at 338 K. Systems linearity is confirmed byreplicate experiments in which the flow rate of n-hexane is variedbetween 1 ml/min and 4 ml/min. Zeolites ZK-5, ZSM-5, ZSM-22, Y,Mordenite (CBV 30A, PQ, Kans., USA) and Beta (PB-1, Chemie Uetikon AG,Uetikon, CH) are ion exchanged with an aqueous solution of NaClaccording to the prescribed method. The resulting Na-zeolite is calcinedat 2 K/min to 373 K for 1 hour and consequently at 2 K/min to 773 K forat least 2 hours. The packed bed of Na-zeolite is calcined at about 1K/min to 623 K for at least 6 hours under a flow of high pressurenitrogen.

The selectivity ratio α_(trans/cis) is determined by interpretation ofthe chromatographic response curves according to the method of moments(D. M. Ruthven, “Principles of Adsorption and Adsorption Processes”, J.Wiley and sons (1984)). A detailed description of the calculation methodhas been described before in the detailed description of this invention.

The selective hydrogenation of trans-unsaturated fatty acid compoundsfrom a mixture containing both cis- and trans-isomers is carried out ina batch reactor. 8 ml of an octane solution of cis- andtrans-unsaturated fatty acids or methylesters is loaded in a 10 mlreactor (home made, KULeuven, BE) and the catalyst, prepared asdescribed above, is added. The reactor is closed, purged with nitrogenand heated. A hydrogen pressure of 6 MPa is applied while stirring themixture at approximately 500 rpm. The hydrogenation reaction isperformed at 338 K. A sample of 0.25 ml is withdrawn from the reactorafter 15, 30 and finally after 60 min respectively and centrifuged toremove the catalyst. The samples are analysed by isothermal gaschromatography at 353 K on a BPX-70 column (SGE, Austin, USA). Whenneeded, samples are derivatised to enable analytical differentiationbetween cis- and trans-isomers. Triacylglycerols are transesterified tomethylesters of their fatty acid chains. To 0.25 ml of the triglyceridesample 1 ml diethylether and 1 ml of a 3 wt. % solution of potassiumhydroxide in anhydrous methanol is added. The mixture is shakenvigorously and after 3 minutes the transesterification is stopped byadding 1 ml of distilled water. Fatty acid methylesters are thenextracted with n-pentane. The organic layer is washed several times withwater and finally dried over molecular sieve (5A, E. Merck, Darmstadt,Del.). Relative sensitivity coefficients of methyloleate,methylelaidate, methylstearate and methylpalmitate are determined withmethyldecanoate as standard. The disappearance, i.e. conversion of cis-and trans-unsaturated methylesters to saturated ones and denoted asX_(cis), (%) and X_(trans) (%) respectively is determined after 60 minreaction time. The first order reaction rate constants of cis- andtrans-unsaturated methylesters denoted as k_(cis) and k_(trans) in h⁻¹respectively are determined after 15 min.

A rapeseed oil is partially hydrogenated with 0.2 wt. % of a supportedNi-catalyst (21 wt. % Ni on silica, Pricat 9910, Unichema, Emmerich,Del.) at 423 K and 0.3 MPa during 150 min in an agitated autoclave.

EXAMPLES Examples 1 to 4

Na-ZK-5, Na-ZSM-5 and Na-ZSM-22 crystals are packed in an adsorbent bedand calcined according to the prescribed methods. The separation factorα_(trans/cis) for these zeolites is higher than 1 and corresponds to thepresent invention. The results of the adsorption measurements,represented in the adsorption equilibrium constants of methyloleate andmethylelaidate and the separation factor α_(trans/cis) are presented intable 1.

TABLE 1 Example No. 1 2 3 4 Catalyst Na-ZK-5 Na-ZSM-5 Na-ZSM-5 Na-ZSM-22Si/Al 2.3 39 77.5 45 crystal size (μm) n.d.* 0.3-0.6 0.5-2 n.d.Adsorption K_(trans) 0.91 3.23 6.09 2.34 K_(cis) 0.88 3.08 5.08 2.21α_(trans/cis) 1.03 1.05 1.21 1.05 *not determined

Comparative Example 5 to 7

Na-Y, Na-Mordenite and Na-Beta are pretreated according to prescribedmethods. The separation factor α_(trans/cis) for these zeolites asdemonstrated in table 2 is lower than 1.

TABLE 2 Example No. 5 6 7 Catalyst Na-Y Na-Mordenite Na-Beta Si/Al 2.717.5 9.9 crystal size (μm) 2-3 n.d. 0.2-0.4 Adsorption K_(trans) 8.770.98 25.94 K_(cis) 9.73 1.03 29.23 α_(trans/cis) 0.90 0.96 0.89

Comparative Examples 8 to 11

The following examples illustrate the catalytic hydrogenation activityand selectivity of Pt/Na-ZK-5, Pt/Na-ZSM-5, Pt/Na-ZSM-22 and Pt/Na—Y asprepared according to prescribed methods. The catalytic hydrogenationconditions as well as the analytical methods for sample analysis arerepresented before. A solution of methyloleate and methylelaidate isapplied as lipid source. The catalyst and substrate composition and thereaction results are given in table 3. The first order reaction rateconstants of methyloleate and methylelaidate are determined after 1hour.

TABLE 3 Example No. 8 9 10 11 Catalyst Pt/Na-ZK-5 Pt/Na-ZSM-5Pt/Na-ZSM-22 Pt/Na-Y topology KFI MFI TON FAU Si/Al 2.3 77.5 45 2.7Pt/lipid (mg/g) 1.42 1.29 1.87 1.32 Lipid (wt.%) methyloleate 4.17 3.993.86 4.20 methylelaidate 2.71 2.76 2.77 2.91 Hydrogenation k_(cis) (h⁻¹)0.032 0.278 0.223 5.537 k_(trans) (h⁻¹⁾ 0.022 0.197 0.149 1.135k_(trans)/k_(cis) 0.66 0.71 0.67 0.20 X_(cis) (%) 2.4 33.8 9.3 95.6X_(trans) (%) 1.8 25.3 5.6 68.1

These examples confirm the influence of the pore size and topology ofthe zeolite on the hydrogenation activity and selectivity promoting thePt/Na-ZSM-5 (MFI) as an optimal zeolite structure. Since no selectivitytowards methylelaidate is noticed, a further optimisation of thecatalyst design is required.

Examples 12 to 14

Following examples illustrate the effect of a finely dispersed metalcatalyst in the zeolite pores on the hydrogenation selectivity ofmethylelaidate. Pt/Na-ZSM-5 is prepared by competitive ion exchange ofPt(NH₃)₄Cl₂ and NaCl with a Na/Pt molar ratio of 25. The obtainedzeolite is washed and calcined according to prescribed methods. Thezeolite samples are oxidised at two different temperatures, 773 K(examples 10 and 11) and 623 K (example 12) respectively. The reactionconversions and the rate constants are given in table 4. The effect ofconditions for preparation of said zeolites is clearly illustrated.

TABLE 4 Example No. 12 13 14 Catalyst Pt/Na-ZSM-5 Pt/Na-ZSM-5Pt/Na-ZSM-5 Si/Al 39 77.5 77.5 oxidation T (K) 773 773 623 Pt/lipid(mg/g) 8.01 8.30 5.44 Lipid (wt.%) methyloleate 0.56 0.52 0.84methylelaidate 0.55 0.57 0.79 Hydrogenation k_(cis) (h⁻¹) 0.247 0.2850.093 k_(trans) (h⁻¹⁾ 0.298 0.892 0.523 k_(trans)/k_(cis) 1.20 3.13 6.25X_(cis) (%) 11.7 20.4 9.9 X_(trans) (%) 17.9 39.9 18.9

Example 5

In the following example a partially hydrogenated rapeseed oil ishydrogenated on a Pt/Na-ZSM-5 zeolite as prepared according to theprescribed method in example 13. Only the mono-unsaturated compounds aretaken into account for the rate constants and the conversions of thecis- and trans-unsaturated fatty acids are illustrated in table 5.

TABLE 5 Example No. 15 Catalyst Pt/Na-ZSM-5 Si/Al 77.5 Pt/lipid (mg/g)9.80 Fatty acid composition* (wt.%) saturated 17.47 mono-unsaturated(cis) 31.23 mono-unsaturated (trans) 49.37 di-unsaturated 1.93Hydrogenation k_(cis)** (h⁻¹) −0.081 k_(trans) (h⁻¹) 0.371 X_(cis) (%)0.47 X_(trans) (%) 15.4 *The lipid concentration is 1.79 wt. % and thefatty acid composition is determined by methanolysis according to theprescribed method. The saturated compounds can be subdivided in laurate(0.08 wt. %), myristate (0.12 wt. %), palmitate (6.44 wt. %), stearate(9.78 wt. %), arachidate (0.66 wt. %) and behenate (0.37 wt. %). Themono-unsaturated compounds are mainly composed of oleate (cis9, 14.51wt. %) and elaidate (trans9, 27.26 wt. %) and positional isomers thereof**Initial formation of cis-unsaturated fatty acids occur

What is claimed is:
 1. Process for the elimination of trans-unsaturated fatty acid compounds from a substrate containing cis- and trans-isomers of said fatty acid compounds, wherein the trans-unsaturated fatty acid compounds are selectively adsorbed by a microporous zeolite material having a selectivity ratio α_(trans/cis) higher than
 1. 2. Process according to claim 1, wherein the microporous zeolite material has a selectivity ratio α_(trans/cis) higher than 1.10.
 3. Process according to claim 1, wherein the microporous zeolite material has a 10-membered ring pore structure and a Si/Me-ratio higher than 39, preferably higher than 77.5.
 4. Process according to claim 1, wherein the fatty acid compounds are selected from the group consisting of free fatty acids, monoglycerides, diglycerides, triglycerides, phospholipids, glycolipids and mixtures thereof.
 5. Process according to claim 4, wherein the fatty acid compounds are triglycerides.
 6. Process according to claim 1, wherein subsequent to the selective adsorption of trans-unsaturated fatty acid compounds, the microporous zeolite material is removed from the substrate.
 7. Process according to claim 1, wherein the selective adsorption of the trans-unsaturated fatty acid compounds is carried out with a continuous substrate supply on a packed bed of microporous zeolite material.
 8. Process according to claim 1, wherein the microporous zeolite material comprises, inside its pores, a hydrogenation metal catalyst selected from the group consisting of platinum, palladium, nickel, copper, cobalt, rhodium, ruthenium and mixtures thereof.
 9. Process according to claim 8, wherein the hydrogenation metal catalyst is nickel.
 10. Process according to claim 8, wherein the microporous zeolite material catalyses the saturation of the trans carbon-carbon double bonds of the trans-unsaturated fatty acid compounds selectively adsorbed by the microporous zeolite material.
 11. Process according to claim 2, wherein the microporous zeolite material has a 10-membered ring pore structure and a Si/Me-ratio higher than 39, preferably higher than 77.5.
 12. Process according to claim 2, wherein the microporous zeolite material comprises, inside its pores, a hydrogenation metal catalyst selected from the group consisting of platinum, palladium, nickel, copper, cobalt, rhodium, ruthenium and mixtures thereof.
 13. Process according to claim 3, wherein the microporous zeolite material comprises, inside its pores, a hydrogenation metal catalyst selected from the group consisting of platinum, palladium, nickel, copper, cobalt, rhodium, ruthenium and mixtures thereof. 